Diarrheal illness is a major worldwide cause of morbidity and mortality, and accounts for approximately 15% of deaths in children. Enterohemorrhagic Escherichia coli (EHEC) and enteropathogenic E. coli (EPEC) are two primary bacterial causes of pediatric diarrhea. The mechanisms by which these pathogens cause diarrheal disease is not yet completely understood, but is initiated when the pathogens colonize the intestinal epithelium (Nataro and Kaper (1998) Clin Microbiol Rev. 11:142-201).
A closely related pathogen, namely Citrobacter rodentium is a murine pathogen that is widely used to model human EPEC and EHEC infection, because mice are relatively resistant to both EPEC and EHEC. In mice, C. rodentium results in colonic pathology that is nearly indistinguishable from that produced by EPEC and EHEC in humans (Borenshtein, M. et al. (2008) Curr Opin Gastroenterol. 24:32-37; Luperchio and Schauer (2001) Microbes Infect. 3:333-40; Mundy, T. T. et al. (2005) Cell Microbiol. 7:1697-706). This may not be surprising, since C. rodentium possesses a homologue of the locus of enterocyte effacement (LEE) pathogenicity island carried by EPEC and EHEC that encodes for the effector proteins necessary for the development of attaching and effacing (A/E) lesions. These lesions are accompanied by the development of colonic hyperplasia, and pathological colitis marked by epithelial detects and leukocyte infiltration (Luperchio and Schauer (2001) Microbes Infect. 3:333-340).
The intestinal epithelium provides a formidable barrier to enteric pathogens. In order to cause disease, enteric pathogens must either adhere to or penetrate/invade host epithelial cells. Thus, interaction with epithelial cells is the first step in pathogenicity for all enteric pathogens, and this step can be studied through the use of A/E pathogens by assessing colonic colonization and resultant pathology.
Colonization of A/E pathogens in the colon is dependent upon the composition of the intestinal microbiota. Inducing dysbiosis (the disruption of the native populations of beneficial bacteria) within the colonic microbiota by administering antibiotics (Wlodarska, B. et al. (2011) Infect Immun. 79:1536-45) or by inducing an inflammatory response (Lupp, M. L. et al. (2007) Cell Host. Microbe 2:119-129) has been shown to greatly enhance pathogen colonization.
Colonic dysbiosis can further exacerbate the inflammatory response to the colonic pathogen (Wlodarska, B. et al. (2011) Infect Immun. 79:1536-1545), but even in the absence of pathogen challenge, dysbiosis can propagate inflammatory responses in genetically susceptible individuals, as evidenced by the findings of dysbiosis in patients with inflammatory bowel disease (Machiels et al. (2013) Gut, published online first Sep. 10, 2013; Morgan et al. (2012) Genome Biol. 13:R79) or irritable bowel syndrome (Carroll et al. (2012) Neurogastroenterol Motil. 24:521-30, e248; Chassard, M. et al. (2012) Aliment Pharmacol Ther. 35:828-838).
Probiotics, are live microbes that when ingested in high enough quantities confer a health benefit for the host (Food and Agriculture Organization of the United Nations and World Health Organization, “Health and Nutritional Properties of Probiotics in Food Including Powdered Milk with Live Bacteria” (2001)), are gaining traction as a viable option for treating enteric diseases (Hemarajata and Versalovic (2013) Therap Adv Gastroenterol. 6:39-51).
Many probiotic microbes have the capacity to enhance immune system activity, but fewer probiotic microbes have anti-inflammatory effects. Lactobacillus reuteri is a commonly used probiotic that has been shown to regulate the mammalian and avian intestinal immune system (Lin et al. (2008) Inflamm Bowel Dis. 14:1068-1083), Studies in vitro demonstrate that some strains of L. reuteri (such as PTA6475) can suppress the ability of myeloid cells to produce inflammatory cytokines (such as TNF-α) through a down-regulation of cell signal transduction pathways (e.g., c-Jun-dependent activator protein 1 (AP-1)) (Jones and Versalovic (2009) BMC Microbiol. 9:35; Lin et al. (2008) Inflamm Bowel Dis. 14:1068-1083).
Other strains of L. reuteri, such as ATCC23272, can down-regulate both cytokine and chemokine production by colonic epithelial cells stimulated with C. rodentium. L. reuteri can also reduce colonic inflammation in both juvenile and adult animals (Eaton, A. et al. (2011) Infect Immun. 79:185-191; Schreiber et al. (2009) Am J Physiol Gastrointest Liver Physiol. 296:G534-G542).
Studies demonstrate that L. reuteri attenuates the exacerbating effects of stress on C. rodentium-induced colitis as marked by reductions in colonic cytokines and chemokines, inflammatory cell infiltration, colonic epithelial cell defects, and pathogen translocation from the colon to the spleen (Mackos et al, (2013) Infection, Infect Immun. 81:3253-3263).
The effects of L. reuteri are most evident when stress leads to mild to moderate C. rodentium-induced colitis. However, under stress conditions that lead to severe C. rodentium-induced colitis, L. reuteri was able to prevent pathogen translocation and the development of systemic inflammatory responses, but it was not able to reduce all aspects of colonic pathology (Mackos et al. (2013) Infection, Infect Immun. 81:3253-3263).
Moreover, the effects of L. reuteri on the host were short-lived and no longer evident after daily administration was terminated. These studies demonstrate the immunomodulatory potential of L. reuteri. 